If they had sailing skills like these, Nemo and Dory wouldn’t keep losing each other.

Unlike most sea slugs that crawl on coral reefs, the nudibranchFiona pinnata lives on the go. These seafaring sea slugs live on floating islands of debris, eating gooseneck barnacles and drifting with the currents. As a result, they span the globe – yet a genetic analysis shows they are still closely related. It seems rafting helps sea slugs find each other.

Advertisement

Although many of these vessels wash up on shore or sink, the species survives as larvae jump to a new home.

“It’s like it’s juggling, the ball is always in the air,” says co-author Jonathan Waters at the University of Otago in Dunedin, New Zealand. “There’s always stuff out there for them to live on. They never need the shore.”

Waters first noticed the sea slugs during a study of kelp rafts, detachable habitats thought to transport organisms across thousands of kilometres of ocean (shown below). Each raft is blanketed with barnacles.

Sea slug life raft

Jonathan Waters

“It’s like this squirming mass of black barnacles on stalks. Some people say it looks really gross,” says Waters. “Then I kept on discovering these other little furry things, these weird looking little bugs in there.”

A colleague identified them as Fiona nudibranchs, giving Waters the perfect idea when Jennifer Trickey, a graduate student at the time and a nudibranch enthusiast, asked for a project. They set out to investigate just how well these animals were circumnavigating the globe.

Trickey wrote to dozens of museums around the world, asking to test their Fionapinnata samples, and collaborator Martin Thiel at the Facultad Ciencias del Mar in Coquimbo, Chile, fished out more from the nearby Pacific Ocean.

Through genetic analysis, they found that Fionapinnata is at least two species, with both lineages covering vast swaths of territory.

The “A” branch lives in temperate latitudes on both sides of the equator, with sea slugs off the coasts of Chile and New Zealand close cousins of those found near Alaska. This could be explained if some temperate sea slugs crossed the tropics, beating the heat thanks to the upwelling of colder water near Pacific coastlines.

By contrast, the “B” branch claims tropical waters.

Warped lineage

Taking to the high seas has given each lineage broader, more connected populations than other nudibranchs, says Waters, showing how natural currents work to disperse animals across large distances.

But outliers in the data also demonstrate how these patterns are warped by humans.

“We had one lineage that was shared by the Azores, off the coast of North Africa, and the North island of New Zealand. To us that doesn’t make much sense,” Waters says.

In this case, the sea slugs may have hitched a ride on ship hulls. Elsewhere, they may be learning to live on plastic.

“There is a very strong natural signal in our data set, but then you think of something like the Pacific Garbage Patch,” says Waters. “Those things all get covered in goose barnacles, and presumably our Fiona nudibranchs get on to those as well.”